A. Field of the Invention
The present invention relates generally to motor control circuits and more specifically to an improved motor control circuit for small appliances utilizing speed adjustment devices controlling an adjustable centrifugal governor device.
B. Description of the Prior Art
In the motor control circuit field, there are various types of governor and solid state devices utilized to improve motor performance and achieve control features.
For example, U.S. Pat. No. 3,701,395 which issued to A.A. Collie on Oct. 31, 1972, describes a solid state control circuit for a motor wherein a centrifugal governor including a pair of contacts is utilized in the triggering circuit of a TRIAC to selectively trigger the TRIAC and thus control the motor speed upon the opening and closing of the governor contacts in response to the variations in the motor speed. In this way, it is not necessary for the governor contacts to carry the full motor current.
Variations of this type of control circuit are illustrated in U.S. Pat. No. 3,317,807 which issued to J. Dorfner et al on May 2, 1967, and U.S. Pat. No. 3,223,911, which issued to E. F. Seiler et al on Dec. 14, 1965. The Dorfner patent describes a capacitive coupling system to the governor to eliminate slip ring connections. The Seiler patent describes the governor control in the triggering circuits of various combinations of switching devices.
The motor control circuit in application Ser. No. 916,189 filed on June 16, 1978 by G. Ponczek and D. Stahly provides a control path independent of the governor device to fire a TRIAC power switching device for a predetermined portion of each AC cycle even when the governor contacts are open. Thus, a minimum voltage and a base power level is provided to the motor each cycle. This arrangement alleviates low speed operation problems and modifies the strictly on-off characteristics of a motor control circuit utilizing only a governor device by decreasing the voltage variations seen by the motor.
While the aforementioned motor control circuits are generally suitable for their intended use and application, they are not totally satisfactory in applications where a wider range of speed control is desired over varying load conditions to operate the controlled motor without hunting or cogging throughout the speed control range and in a smooth continuous manner.